1. Field of the Invention
This invention relates to an improved uninterruptible power supply typology, and more particularly to typology that reduces the cost of uninterruptible power supplies.
Uninterruptible power supply circuits are well known and widely used in the prior art. One class of prior art uninterruptible power supplies has a front end stage for converting an a.c. input, such as public utility input, to a d.c. rail voltage. A pulse width modulated invertor converts the rail d.c. voltage to a regulated a.c. output voltage. A normally open switch couples a back-up d.c. supply (e.g. a battery) to the invertor input. In the event the a.c. input fails or falls below a predetermined level, power is supplied on a continuous basis from the back-up battery.
2. Description of the Prior Art
A specific prior art proposal for a universal power supply typology of the class outlined above is disclosed in U.S. Pat. 5,126,585 to Boys. The Boys typology is advantageous in that it provides a high d.c. rail voltage to the pulse width modulated invertor, reducing the current capacity for which the invertor switches must be rated for a given output power. In addition, the Boys typology provides a ground or neutral common to both the a.c. input and the a.c. output. However, the Boys typology is costly to implement due to the number of components it requires.